Device for preventing electrical ignition of stored inflammable fluids



BEST AVAILABLE CCP;

I 1,617,788 Feb, 15 1927i L, BALDWIN M- DEvlc'EfoR PREvEyTING ELECTRICALIGNITIoNf o? s'ro'nED INI'LQMMABLSFLUIM t Fiied nay 25, 1925 4sheets-shut 1 Feb. 15, 1921.

51:6 l AVAILABLE CUF" l.. A. BADwlN ET AL DEVICE FOR' PREVENTINGELECTRICAL IGNITION OF STORED 'INFL-AJIHABLE TTJUIDS Filed May 25A. 192514 sneetssnqet :s

"n 'E f@ BEST AVMUBLE COP\ I 1,611,788 Feb l5 1927' l.. A. BALDWIN ET ALnavlcs son Pnsvsx'rms. ELECTRICAL IGNITION oF sToREn :NFL/mumps Fwzv vFiled May 25', 192s'. 4 sh'ets-snet 4 am, Kwam/ABLE COPi Patented Feb.15,1921.

'juNiTEosTAriis l ...-...w AwoLABLE-COF ori-ien LESILIE A. BALDYV'HV,'UNIVERSITY CITY, HISISOUBI, AND ELMEB E. SCAEFZFEB, Ol'

CBESTWOOD,

YOBX, AS8IGNOBS TO JOHNSMANV`ILLE INCORPORATED, OF

NEW YORIB', N. Y., A CORPORATION OF NEW YORK.

IJEvICE Eon'lPnEvEn'rINo ELECTRICAL IGNITION or s'roEEn man lApplicationled Hay 25, 1925. Serial No. 32,738.

This invention isl concerned withthe preven'tion ofpreventable fire and.explosion risks from electrical ignition of naphthas,

oils,- hydrocarbon 'gases and other inflam- -iiiiiiiiiy i0 .explosions@rares iniaaieaby lating coverings are not'easily penetrated by .sparkdischarges, they are not wholly inielectrical disehang'es: Petroleum.its deriva.

tives, va'pors and. associated gases are good insulators; the tanks usedfor storage and handling are 'generally metallic. necessarily`open"to`thfatmosphere at one point at least usually also',"-fatf1ma'nyaccidental `vp'o'ints)`: "and Athe structure of these tanks dictatedby-'custom or by intelligent econ- 'muy does not avoid providing` idealopport/unitylfor electricVV spark discharges between parts of'thestructure through combustiblessuch as inxeda'ir and vapor or gases. Mostof the oilf'ields are in regions of high probable occurrence ofisolated, lightning discharges and of thunderstorms, high winds` largediurnal variations of temperature, and other meteorological conditionslending to high surface elei-trifications aswell as'to ex-A posiire tothe vast suddenfdiii'erences of potential incident to lightningdischarges. pre- Vlightning stroke electritications. and the in' f ducedfiows and discharges following nearby lightning flashes.

In modern practice heat insulation of stor` .age tanks l'iyproltectivecoverings is more and more resortedio. In Vthe best practice. suc'hstructures arecharacteristically also of dielectric siibstiiinceili.Welt adapted to increase the mutual electrostatic apacity'ot metallico'r,A moisture-filrn` surfaces between which they.l may intervene.lVliile such insumune to puncture by discharges fi'onrsurfacejtnlsurface of the Leyden-jar type of condenser which they oftenconstitute, and they do not' oppose any adequate obstruci tion,v ofcourse. t discharges` either direct or secondary, of lightning strokeintensities.

High static potentials'on the outer surfaces of insulating covers, paintor other coatings shouldbe avoidedbecause they connote inducedrpotentials on surfaces at or near which sparking would ,be ,`d isas,trous, andindicate the possibility of penetrating sparks ordisruptive .or sudden discharge, or sudden augmentation, of the'exterior charge. It is desirable to prevent, therefore, the

building-up .to high. potentials of electro-` staticl charges ofOopposite sign across the insulation vcovers lor fabrics of such tanks.

j It is desirable to avoid every poible opportunity for` a sparkdischarge through or across-spaces likelyto contain air and vapormixtures, as well'as to provide, in optimum relation'to the containersto'be protected, sound metallic :conducting paths to earth capable ofdealing with current surges of great 'quantity land 'high inten/sity.Provisions-to these ends should be of sufficient effective reliabilityand durability to defy being put vinto an inoperative state by ignoranthandling-` but they should nottake up undue space or be' too costlyiorgeneral adoption in and about oil-Wells, tank farms and refineries. K

Since the times of Franklin and -Faraday lightning prevention deviceshave been provided rather as the effect of iiigenuous faith than as theresult of exact engineering knowledge. Lightningstroke conditions remainin part 'a mystery' of physical science, but nevertheless a sufficientbasis of established fact is in hand to indicate and predict suc-- cessor failure of preventive devices upon exposure to lightning .riskplieric electrification phenomena of normal characters andintensities.'A This invention seeks to provide optimum protectionagainst normal risks for such exposed objects' as oil tanks within apracticable minimum 4of coniplexity and cost.

tion of high static potentials between the .exterior-'surfaces andinterior structures of heatinsulated tanks, to

avoid differences of potential between internal parts of tanks,`

gradients abnormally steep in comparison with the potential gradients atV'nearbyobjects, buildings and-surfaces of the earth or' earthedsurfaces. The Ainvention will be explained in conneet-i011 with typicaland recommended constructions illustrative .of the various' arrangementby which it may be carried out.

In the accompanying drawings of the lform-shown forpurposes ofillustration,

Figgl isa diagrammatic. plan view of a 'quadrant Vof a tank showing asystem of conductors; I

Fig. 2 isa diagrammatic vertical. sectional view on' a.. radius of thetank;

Fig. 3lis a side elevation and Fig. 3 a

I plan ofk acentral'psupport. for.. certaincon.-`

Fig. 4-is` a side` elevation of vone of the peripheral supports inposition on the side of a tank;J A

Figs. 5- arid 6 are respectively side elevationsand plan views of aholding clamp;

Fig. 7 isl a ,sectional view onl theline 7- -7 of Elg. 4; Y

Fig. 8 is fa sectional view on the line 8,8 ofFig. 4;

Fig. 9 :s adiagram illustrating the dis-v tribution of equi-potentialsurfaces in a typical vertical plane of a tank between .earth and anelevated atmospheric layer,

such as a cloud;

F ig. 10 is a similar diagram of a detail.

of Fig. 9 showin equi-potential surfaces between and near idleprotcctivesy'stem and the tank surfaces; and

Fig. 11 is a diagram in radial section illustrating the distribution otinduced static charges between the tank top. a heat-insulating cover,and the protective system.

Referring now to the drawings, .let it be assumed that the structure. tobe protected is a tank, which has a peripheral wall 2 built up ofcourses of Steel platesand a nonametallic .heatin'sulating top 4 makinga sealed joint at 5, Fig. 4, with the Nall 2; for example, of theimproved construction described and claimed in the application forproviding BESTA i VAlL-fgpv patent of Leslie-A. Baldwin, Serial No.675,146, tiled November 16, 1923.

As shown, upon the Centex` of the tank. roof a suitable support iserected, which may comprise a rectan ular wooden ost 6, suitably bracedby mem ers 7 and ma ing a water-tight -oint with the cover 4 with theaid of suita le fiashings Siembedded in a waterproof cement. At its uper end the post 6 is provided with a snug y fitting galvanized metalcap 9' and this is surrounded by and supports a two-part ring member 10to which the central portions of a protective conductor'system may beattached.

At the periphery of the tank there may be arranged a spaced series oferect peripheral conductor and supporting members 12, which members maybe made up of a pair of angle-irons 13 of comparatively light sectionbent, as indicated at 14 in Fig. 4, at

their lower ends and fastened together suitably at points 34. At thelower extremities of the members 12 the angle-irons 13 may embrace and'be attachedi to a plate 1omounted between a pair of vertical angleirons16 of slightly heavier section to which th; plate 15 may be riveted orotherwise xed. At a substantial distance above their lower ends themembers 12 are secured as by means o rivets and plates 17, to a pair ofhorizontally extending anfrle-irons 19 and the latter at their innerendsembrace at opposite sides a plate 20 in turn fastened between the-upper ends of the angle-irons 16. The structure so formed is adapted tobe attached to the tank 1 in any suitable way a good mechanical andelectrical Contact with the vertical tank wall. example, at the cxtremeupper and lower ends of the, members 16 T-seetions Z1 andlZ respectivelyare arranged, these being riveted to'the members 16 to providerelatively flat surfaces :23 and 24 against which tension cables 29, 30.may bear. The members 23 and 24 may be drilled vas indicated at 25, Fig.8, to permit thepa5sage through them of the threaded extremities 26 ofU-holts 27, for example, which provide a fairly close tit for the heavywire tension cables 2S), 30 surrounding the tank, and adapted to bestretched taut by turn buckles 31 and 32, Fig. 2. The bearing surfaces'between the parts 1z l1nd the trink wall 1 may be scraped and cleanedbefore erection and protected by applied paint or cement after erection,in order to preserve dependable areas of actual conta-ct, und any otherfastening means for the erect conductorsl 12 capable ofi convenientconstruction and safely holding the con ductors 12 against wz-ithing andexpansion motions of the tank and maintaining electrical contncti'may beresorted to. Suitable lclamp connections 35, for 'example havingportions forming a loop 36 and provided tl AVAILALE COPi with tihteninen 88 in .t e mem the latter, as shown-four of these-connecl.tionsbeing provided on each of the members 18, the first directly op oeitethe periphery 'of the tank cover-and t e second and third `43 of,whichupper conductor may be a strongI wire cable-43 spiaced substantiallyabove thej top ofthe tan for example at a l :height of 'about 7 feet,and defining a periphery somewhatgreater than that of the tank. Betweenths'member 43 andthe ring member there may extcndfany sutiicient numberof substantially radial conductor ele ments 44, the latter at theirouter ends en-y gaging the'conductor 43. lAt points 4Z approximatelyone-third-of the way from the rin Y 10 to the meinber.43 there may beati 'tac ed to each ofthe elements 44 a pair ofwire conductor elements45, which are con-V 1 galvanizedv wire cable.

nected to the peripheral .conductor 43at equal distances ong-fo positesides of the terminal portions of t e conductors 44. 'lhe vv-wirea4,5{may' be twisted around the wires 44 andthe-joint may beheld by anysuitable type Aoffpoeitive clam' The n1embera 45 may be twisted arounthe member 43, so1 dered orclamped; referably all wiring'eonnectionajaresaitab y. recuredbyhindngscicw clamps of anysuitableconstruction.

' Ah forf tie. various cables and conductor elements; it is satisfactoryto provide galvanized steel cables 29, to hold the members l2 in fixedposition upon the' tank, and ordinary galvanized iron wire of sullccntsize may he used-for all the other conductors. 'lhelxfripheralconductor' 43 is desirably a The lower ends of the members 16may advantageously be allowed to rest against. the top edge of the nextto the top course of plates in the construction of the tank wall.

The tank wall 2 is, as usual, in reliable .electrical contact with' theearth.. When this is -not assured by pipe line connections or standingfoundation water. pnoper earthin'g` conductors are provided. Unnecessaryproject ions above the tanktopare rcconuncndwl to he avoided,

ats 37 are' placed in holes re 13 at various pointe in suitable materialmay be .employed i' fined by them from the roof surface. Theserecommendations -low .from the conditions under which the addition ofthe protective conductor system to the tank resultsy in a redistributionof the equipotential surfacesl 70 -related to induced charges, for thefollowlng `reasons:

Assuming an elevated electrified area,such as a-charged cloud C, Fig. 9,directly-overhead, the normally dielectric air between'l earth and cloudis in a state of stress between the oppostepotentials of earth andcloud. This condition results in a gradient of potential differences,of. which equal values will define equipotential surface?1 c1, ege, SQ

etc., the distribution of whichI fol ws in the main the depressions andelevations of the surface of the earth and conductive objects on theearth. Potential gradient intensities of the order of 35,000 t-o 150,000volts per Q foot of elevation 'are 'of common occurrence during,rthunderstorms, and the potential gradient to upper-strata'may approach-asubstantial fraction of these values during Aweather conditions notdelinitely of the na. 90

ture of a thunderstorm. l.

In the vcase of a cloud or charged stratum .net overlwzul, theequi'poten-tial surfaces are vconcave toward the c tuin, and thelLradlent intensities may be of iai-ged 4elevated straany lesservalues-dependent ou. the lateral distance. of thecharged ftratlun. Ineither casetthel lilies Uf force representing Athe `path 'of dischargeand indicating;r the -norma s to tlu` equipotential surfaces are notnecessarily vertical. .()n the contrary, in the case of a shariterrestial elevation, such us a tree, a luiildinpy or a tank. there is acompression to: gelber of the equipotential sui-faces at th tips` edgesand -projections of these objects;-

this is inert-,ly another way of saying that at these placesthepotential gradient is steeper, and that the directions ol the linesof force are incliped 'toward the center of Aligure of the terrestrialprojection as indicated at f1,

f2, f" in Fig. t). Wlien there is normal uniformity of the atnrosphere,it may he predictedthat a lightning stroke discharge will strike alongthe steepest gradient 'and through one lof these compressions of the 315equipotential surfaces. When 'thereI is no stroke, thel inducedelectrostatid vcharges are of greatest-intensity' at there places.

The linesof force end at. any most. elevated part, as-measured alongthe. direction of the nes ofA force, of the terrestrial projection, whenthat part is competently conductive'to earth. ln the case'ota suspendedearthed' conductor, such as the wires 43, 44, 45 of .the describedapparatus, the place of coiu- .1

pression together of the equipotentialsur# faces is elevated away fromthe tcrrestial projection. (the tank in this case) and if of anysubstantial electrostatic capacity, or'if the c'onductor bearsV a movingcharge or eur- 'series of cquipotentialsurfaces of reversed rent, willthen be "the" starting point of a .rent corresponding to static chargesinduced upon any such systeuias a whole are localized in the surfacesrepresented by an intervening earthed conductive networ so spacedaway'from the earthed Vprojecting We have determined by new research'the proportional protective etiect of the descri `ed svstem ofspacedradial and periph.- eral conductors under high potcntialgradients. Thedistribution of equpotential surfaces (altered by the additions to thetank as shown in Fig'. 9 in full lines) in one plane is'indicated by thetraces p.' 7i?. 7i etc. in' Fig. 10,without attempt toindicate the.ahsolute intensities from plotted surface to surface. When the meanlateral spacing` from Vconductor to conductor is equal to -or less thanthe verticalspacing from the tank roof to the conductors, the inducedflux of current (a. direct measure 'of the static charge induced charges uch isolated surfaces by direct by theeirciting charged stratum) wasfound -to be proportionally distributed between the tankroof and theconductor system in a proportion between extremes represented bytheobservedextreme'values 3.2 to 96.8 and 1.6. to 98.4. When the number otradial conductors wasreduced 'so that the mean separation of theconductors was greater than their elevation abovejthe tank4 roof,thcn'the inean values of'tlie. proportional charges were as-14.3 isto85.7. In'this ease-e iupotentialv surfaces between .the suspen edconductors and the rootl intersected the roof .at the iiiore separatedplaces between the. conductors.

. The protective system, in the recommended spacing. -eiirries anaverage value of' the total induced charge of 97.6%. AThis may be takenas a probableniininium evaluation oitlie proportion of the current of alight- `ning holt discharge likely to be carried by the'protectivesystem an( the tank roof rcspect-ively in caso of a stroke toa protected tank. The relativel high conductivity and even distributiono the conductoi' paths to earth from the protective covering warrantsbelief that a -far greater pro oi-tion of a heavy discharge'than theinduced charge ratio indicates would in that case tlo'iv in theprotective devices, resulting in a practically complete exclusion of theischarge currents from the tank top androof proper.

Referring now to Fig. 11, the recoin-- mended proteetive conductorsystem is also Aand top surfaces, separate BEST AVAiLABLE-COP- 'chargefrom part to part of the tank. The

probability of inter-part sparks increases as the potential or intensity6i mutually-in duce( or bound charges is permitted te increase. (and toa lesser degree paint or other coitings) will provide exterior surfacest interior surfaces R which, because conductire or'anade conductive bycondensed mois'- ture, residual -sulphuric acid fumes or other thintiliiis, 'may provide a path for a-static charge built up' todischarging intensity.

Tivo such surfaces, especiall the large roof by a felt, as-

'lhe heat insulation I of the tank.

plialt or paint. dielectric I arei'an ideal Leyden-jar condenser capableof energetic d1s` charges 'amply able to fire the inflammables. lVorking)oints in the surface l, for example, under expansion or fillingwrithings of the tank, might, if this surface (and the sui-face t byinduction) carried heavy in- 'duced charges. permit local sparks frompart to part: :is illustrated .at '1-1, sections'of the i tank,especially the eaves portion, may re resent a resistance to ground (dueto o structcd joints,-openings at the eaves, etc.) and the moisture filmat t may "well be electrically discontinuous or highly resistant, as ati. Y c .l Y

There is no good way to reach and discontact wiring system could bemade. lVe

therefore. prefer to rel),r for discharge upon the overhead conductorsystem 12, 44, spaced from the surface t by an air-gap dielectricresistance. The overhead conductors coinpletc a system in which thetypical assumed 'charges at preferentially discharge to earth throughthe conductors 44, 39, 40, 1 2 and through the resistance r2. ratherthan by penetrationiot l. Such charges at t, when that surface isconductive, arc of the saine sign as the earth; hut as`indicatcd. whenthe surface is not uniformly conductive oi' is conductive in isolatedareas only, these charges may he negative or positive. oi negative inone place and positive in another. or iii a state ot' oscillation tronione to another relative sign ot potential. 'l`he' discharge of suchcharges to thc overhead conductors is gradual rather than disruptive.Dangerously high potentials therefore do not build up on the isolatedsurface or surfaces t, and hv induction on the opposite surface R; and,

it and when a dangerous potential in sip-- preached. this is discharged'without heavy ioo llti

rio

-eesmiiiiieaetecow flow to the conductors 39, t07 41 or 44. It will beobserved that the spacing of theconf ductors mutuallyand from theinsulated .surface of the top of the tank is of optimum '5 'fnnctio lvalue when ythe relations are such *as both to discharge' surface staticcharges and to elevate regions .of high potential vgradient away fromthis surface.

During times of' great lmeteorologi(al lec- Il trical disturbance, "vettectofthe protective rconductors is aided by'- the' corona dischargeionization "phenomena at the conductors, which then carry-'hea currentsat gizeat potentials and u liigl'iifosc'l freqiiencies.- When it lisb'or'ne inl-.mind that theelfect of.l the pro- -tective gridis to lessenthe charges on the tank proper to only from 1.6 to 3.2 per centuin. ofthe .quantum otherwise induced ,l

.flammable fluids having u vapor and beat in- :o by the charged cloud or elevated stratum, it will be perceived thatthe protective sysv temspecified may be reliedvupon to-reduce tlieprobabilityof inducedinterior sparking from condenser action 'of .lms borne by heat zinsulating or paint coverings to a minimum, while providing@ goodprobable protection for lightning-stroke disaster as it is f possible todesi l 1. The combination with a tank for inflammable fluids exposed tometeorological electrical influences and having an insulat ing layerabove a portion of its extent of a sys moi earthed conductors spacedfrom .Il -each other and spaced from the surface of insulating layer byagreater distance than theirmean separation from each other.

2. The combination with a tank for in' flammable fluids exposedtometeorological i electrical influences and having an insulating, layerabove a rtion of its extent of a System ofearthedgonductors, spaced fromeach other and spaced from the surface of said iii-'- sulating layer,si. l conductors being principally concentrated by closer spacin at theregions of maximum potential gradient ./between the tank top and anelevated charged atnios heric stratum. l

3. lllie combinationwvith a tank for. inflammable fluids exposed tometeorological potential gradient lying about and above the p peripheryof the tank top and between the tank top and an elevated chargedatmospheric stratum. f

4: The combination with a tank for mflammable 'luids exposed tometeorological electrical influences and having, a dielectric laversuperimposed upon' its top of a system of earthed conductors extendingbeyond the the .surface-charge dising layer above a portion of itsextent con- I-prisin electrical intluences of a system of earthed 'iperiphery of the tank and spaced from'cach other and from the topportion of vthe tank by erect earthed conductors, said conductors beingprincipally concentrated in the regions of maximumpotei. '.ial gradientbetween the tank top and an eievated chargedl atmospheric stratum.' i 5.The combination with a tank for inlariimable'ifluids exposed tometeorological electrical `influences and having an insulat stitiitiiigfi' dielectric separating outer andv inner snrfaces, of vmeans fordissipating inl duced s'tatic chai-ges held by mutual attraction atopposite sides of said dielectric coma system of eartlied conductors'space by an air-gap from tlie'exterior of the insulating layer.

6. The combination with a tank forinsulating top exposed tometeorological electrical 'influences of an interconnected. sys-- tem-ofconductors spacedfrom each other and from the top portion of the tankand com rising a series of erect supporting and con uctingniembersin-earthed contact with metallic sidewalls of the said tank, one or moreperipheral conductors connecting said erect members and encircling theregion of the eaves ofA the tank, and a 'series of con- 95 -ductorsextending from the region of the center of the to .of the tank toone ofsaid peripheral con uctors and sup rted above the tank top at a distancegreati); than their mean separation apart. -f` 10 7. The combinationwitha tank for inlflammable fluids exposed to meteorological electricalinfluences andliavingan insulating layer above a ortion of its extentconstitutl ing a dielectric separating outer and inner 106 surfaces, of`means for dissipating induced static charges held b mutual attraction atop site sides of sai .dielectric comprising a system of eartbedconductors space by an Aair-gap from the exterior .of the insulating.110 layer, at' a distance at which the air-gap dielectric resistance isnot eomtantly lgreater at times of atmospheric electric: disturbancethan the dielectric resistance of the said insulatn layer. 116

8; T e" combination with ametallic stora e tank, and a roof thereforhaving low eactrical conductivit of a protective system 'comprising conuctors overlying said tank and supporting means therefor includ- 120 ingsupporting elements projecting beyond and above the periphery of saidtank and means for holding the snpportingelements in positionincluding-flexi e members holding their lower ends in mechanical 'andelec- 125 trical contact with the walls of said tank below sajd roof,

'9. The combination A with a metallic storage tank, and a rooftherefor'having lowr electrical conductivity, of a protective sys- 13,0

, ductors overlying said tank, and supporting point well'below theeaves.

BEST AVAILABLE COP.

B Y A 1,617,788

temlcomprising a series of connected eon-y electrical Contact with thetank Walls'at a Y i0 means therefor including a,` plurulityof erectlvSigned by me at St. Louisliissorl1i, rliis conductive elementsprojecting upwardly fifteenth day of May 1925. and outwardly from thewalls of said tank,- LESLIE A. BALDVv'lN.

'and holding means therefor including a plui Signed by me at Boston,Massachusetts,

rality 'of tension elements `adapted to hold this 22nd day of May, 1925.the said erect conductors in`meehanieal and ELMER R. SCHAEFFER.

